fix(wpcarro/nix): Remove <briefcase> references

Angle-bracketed references are a Nix anti-pattern, and thankfully this
repository enforces this as a standard.

TL;DR:
- Drop angle-bracketed references
- Change `briefcase` -> `users.wpcarro`
- Fix any resulting regressions
- Fix //users/wpcarro/tools/simple_vim
- Mark //users/wpcarro/boilerplate/typescript and related projects
  as broken
- drop .skip-subtree file, enabling depot CI

Change-Id: I7153cbabafa617bfd6b199370cbec65cb75441f6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4325
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: wpcarro <wpcarro@gmail.com>

users/wpcarro/emacs/.emacs.d/snippets/nix-mode/shell-nix

@@ -2,12 +2,11 @@
 # name: shell.nix boilerplate
 # key: import
 # --
-let
-  briefcase = with import <briefcase> {};
-  pkgs = briefcase.third_party.pkgs;
-in stdenv.mkDerivation {
+{ pkgs, ... }:
+
+pkgs.stdenv.mkDerivation {
   name = "$1";
   buildInputs = [
     $2
   ];
-}
+}